Natural rubber, also called India rubber or caoutchouc, as initially produced, consists of suitable polymers of the organic compound isoprene, with minor impurities of other organic compounds plus water. Forms of polyisoprene that are useful as natural rubbers are classified as elastomers.
The main constituent of NR is cis-polyisoprene, with a molecular weight of 100,000 to 1,000,000 daltons. It is the combination of the high cis content, the high average molecular weight, a minor but essential amount of branching and the relatively broad molecular weight distribution that give natural rubber unique properties. The feel and touch of dipped goods like condoms and surgical gloves are much appreciated.
Some people have a serious latex allergy, and exposure to natural latex rubber products such as latex gloves can cause anaphylactic shock. For this and other reasons dipped goods made from synthetic rubber based on polyisoprene are popular.
For instance, Cariflex™ polyisoprene products are the ideal alternative to natural rubber for applications that demand extreme purity, exceptional protection and consistently high quality. Cariflex products, supplied by Kraton Polymers offer a pure and versatile option for manufacturing applications that require the high tensile strength and tear resistance of natural rubber without the impurities that cause discoloration, odor and allergic reactions.
When made by anionic polymerization, polyisoprene is made without the allergens. The molecular weight is relatively high as is the cis content. However, in anionic polymerization a relatively sharp molecular weight distribution is achieved (Mw/Mn of about 1.0) and a nearly linear polymer structure is obtained. To achieve the same properties as NR, some broadening of the molecular weight distribution (wherein the expression “broad MWD” refers to Mw/Mn greater than 1.0, preferably at least 1.1, more preferably at least 1.5) and some branching is necessary. On the other hand, gelation (cross-linking) should be avoided. It has been found problematic to achieve these requirements when preparing a conjugated diene polymer such as polyisoprene by anionic polymerization. Polyisoprene may also be made by other processes, but then the polymer is neither clear nor free of allergens.